In several industrial applications, components having through-holes need to be inspected for proper functioning of the component. One such example is a turbomachine blade or vane. Turbomachines, particularly gas turbines, have vanes and blades that contain channels or through-holes which lead from an exterior surface of the blade or vane to a hollow interior (plenum). In use, pressurized air is applied to the plenum causing cooling airstreams to flow through the through-holes, thereby absorbing heat from the walls of the through-holes, as well as from the surface of the blade or vane, thereby cooling the blade or vane.
In order to function properly, these through-holes, called cooling holes, must be made to a known configuration because the distribution of airflow must be controlled in order to achieve proper cooling. Accordingly, the cooling holes must not be blocked, not even partially. This requires a reliable technique for inspection of the cooling holes to detect blockages from the external surface. If a cooling hole is detected to be blocked, even partially, it may require the component (i.e., the blade or vane) to be discarded and replaced.
Currently checking and inspection of cooling holes of blades or vanes of a gas turbine is done by using infrared thermography. Herein, a medium, generally a hot gas, is forced through the cooling holes and an infra-red camera is used to capture infra-red radiation emitted by the component while the medium is flowing through the cooling holes, to form a thermographic image. Blocked cooling holes obstruct the flow of heat, which is visible in the thermographic image.
Thus, to decide whether or not the component can continue to be used, an evaluation of the thermographic image is required. Currently, the evaluation of thermographic images to detect blocked cooling holes is done manually by trained personnel. This requires individual contrast adjustments for the individual thermographic images, recognition of irregularities in pattern of the cooling holes in the component, counting the rows of hot spots (which correspond to cooling holes) in the thermographic image, and a comparison with the cooling hole pattern in the actual component. Often, it is also necessary to insert a diameter measurement gage or a pin into a cooling hole to inspect irregularities.
Further, evaluation of the thermographic images with standard image processing procedures is not possible because of high noise components in these images and also due to inhomogeneous illumination in the thermographic images due to curvatures in the component.